Active Mercury

April
30, 2009: A NASA spacecraft gliding over the surface
of Mercury has revealed that the planet's atmosphere, magnetosphere,
and its geological past display greater levels of activity
than scientists first suspected. The probe also discovered
a large impact basin named "Rembrandt" measuring
about 430 miles in diameter.

These
new findings and more are reported in four papers published
in the May 1 issue of Science magazine. The data come from
the Mercury Surface, Space Environment, Geochemistry, and
Ranging spacecraft--MESSENGER for short. On Oct. 6, 2008,
MESSENGER flew by Mercury for the second time, capturing more
than 1,200 high-resolution and color images of the planet.

Right:
The Rembrandt impact basin discovered by MESSENGER during
its second flyby of Mercury in October 2008. Credit: NASA/Johns
Hopkins University Applied Physics Laboratory/Smithsonian
Institution/Carnegie Institution of Washington. [more]

"This
second Mercury flyby provided a number of new findings,"
said Sean Solomon, the probe's principal investigator from
the Carnegie Institution of Washington. "One of the biggest
surprises was how strongly [Mercury's magnetosphere] had changed
from what we saw during the first flyby in January 2008."

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The
magnetosphere is a region of space around Mercury enveloped
by the planet's magnetic field. Gusty solar wind buffeting the
global bubble of magnetism can potentially trigger magnetic
storms and other space weather-related phenomena.

"During
the first flyby, MESSENGER measured relatively calm dipole-like
magnetic fields close to the planet. Scientists didn't detect
any dynamic features other than some Kelvin-Helmholtz waves,"
said James Slavin of NASA's Goddard Space Flight Center. Slavin
is a mission co-investigator and lead author of one of the
papers.

"But
the second flyby was a totally different situation,"
he says. MESSENGER observed a highly dynamic magnetosphere
with "magnetic reconnection" events taking place
at a rate 10 times greater than what is observed at Earth
during its most active intervals. "The high rate of solar
wind energy input was evident in the great amplitude of the
plasma waves and the large magnetic structures measured by
the spacecraft's magnetometer throughout the encounter."

Another
exciting result is the discovery of a previously unknown large
impact basin. The Rembrandt basin is more than 700 kilometers
(430 miles) in diameter and if formed on the east coast of
the United States would span the distance between Washington,
D.C., and Boston.

Rembrandt
formed about 3.9 billion years ago, near the end of the period
of heavy bombardment of the inner Solar System, suggests MESSENGER
Participating Scientist Thomas Watters, lead author of another
of the papers. Rembrandt is significant, not only because
it is big, but also because it is giving researchers a peek
beneath the surface of Mercury that other basins have not.

"This
is the first time we've seen terrain exposed on the floor
of an impact basin on Mercury that is preserved from when
it formed," explains Watters. "Landforms such as
those revealed on the floor of Rembrandt are usually completely
buried by volcanic flows."

Half
of Mercury was unknown until a little more than a year ago.
Globes of the planet were blank on one side. Spacecraft images
have since revealed 90 percent of the planet's surface at
high resolution. This near-global coverage is showing, for
the first time, how Mercury's crust was formed.

Right:
In this interpretive map of Mercury's surface, shades of yellow
denote smooth plains of mainly volcanic origin. This type
of terrain covers approximately 40% of the planet. The white
(empty) slice is the portion of Mercury not yet photographed.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Arizona
State University/Carnegie Institution of Washington. [more]

"After
mapping the surface, we see that approximately 40 percent
is covered by smooth plains," said Brett Denevi of Arizona
State University in Tempe, a team member and lead author of
a paper. "Many of these smooth plains are interpreted
to be of volcanic origin, and they are globally distributed.
Much of Mercury's crust may have formed through repeated volcanic
eruptions in a manner more similar to the crust of Mars than
to that of the moon."

Another
finding of the flyby is the first detection of magnesium in
Mercury's exosphere. The exosphere is an ultrathin atmosphere
where the molecules are so far apart they are more likely
to collide with the surface than with each other. Material
in the exosphere comes mainly from the surface of Mercury
itself, knocked aloft by solar radiation, solar wind bombardment
and meteoroid vaporization:

The
probe's Mercury Atmospheric and Surface Composition Spectrometer
instrument detected the magnesium. Finding magnesium was not
surprising to scientists, but the abundance was unexpected.
The instrument also measured other exospheric constituents
including calcium and sodium. Researchers believe that big
day-to-day changes in Mercury's thin atmosphere may be caused
by the variable shielding of Mercury's active magnetosphere.

"This
is an example of the kind of individual discoveries that the
science team will piece together to give us a new picture
of how the planet formed and evolved," said William McClintock
of the Laboratory for Atmospheric and Space Physics at the
University of Colorado at Boulder. McClintock is co-investigator
and lead author of one of the four papers.

"The
third Mercury flyby [coming up on Sept. 29th] is our final
dress rehearsal for the main performance of our mission, the
insertion of the probe into orbit around Mercury in March
2011," said Solomon. "The orbital phase will be
like staging two flybys per day and will provide continuous
collection of information about the planet and its environment
for one year."

"Mercury
has been coy in revealing its secrets slowly so far, but in
less than two years the innermost planet will become a close
friend."

The
MESSENGER project is the seventh in NASA's Discovery
Program of low-cost, scientifically focused missions.
The Johns Hopkins University Applied Physics Laboratory
of Laurel, Md., designed, built and operates the spacecraft
and manages the mission for NASA's Science Mission Directorate
in Washington. Science instruments were built by the
Applied Physics Laboratory; Goddard; the University
of Michigan in Ann Arbor; and the University of Colorado
in Boulder. GenCorp Aerojet of Sacramento, Calif., and
Composite Optics Inc. of San Diego provided the propulsion
system and composite structure.